= Solution
\b[<water> achieves substantial binding through a localized O–H···O <hydrogen bond>; <benzene> achieves a comparable total through a larger set of weaker contacts.] In the <water dimer>, one <water> is the <hydrogen>-bond donor and the other presents an <oxygen> <lone pair> region as acceptor. The interaction includes <electrostatics>, <induction interaction>, dispersion and short-range <exchange repulsion>; a <hydrogen bond> is not a separate fundamental force that can simply be added to all those terms again.
For <benzene>, draw a T-shaped edge-to-face pair or a slipped parallel pair. Its larger, more polarizable <electron> clouds provide appreciable London dispersion. Its quadrupolar charge distribution also makes the energy orientation dependent, despite each <molecule> having no permanent dipole by <symmetry (physics)>. An edge C–H approaching the other ring's pi region is therefore preferable to a model of two structureless, noninteracting dipole-free objects. Dispersion attracts while <Pauli repulsion> limits close overlap. Several distributed contacts can sum to the scale of one <water>–<water> <hydrogen bond>; counting only the number of conventional <hydrogen bonds> would miss them.
The comparison concerns the given gas-phase pair interaction energies, not an equality of every <solvent>-dependent association free energy. Correlated primary calculations explicitly identify both T-shaped and parallel-displaced <benzene>-dimer minima, rather than a uniquely preferred perfectly eclipsed sandwich.<Estimates of the Ab Initio Limit for π–π Interactions: The Benzene Dimer by Mutasem Omar Sinnokrot and others>[2002, abstract and pp. 10887–10893, comparison of the three dimer geometries]{ref}
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-1-dimers.png]
{title=Original schematic geometries of a hydrogen-bonded water dimer and T-shaped and parallel-displaced benzene dimers}
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